My Chev Impala 2001 base (57K) has been making hissing noise at
acceleration. The noise came into the cabin from front somewhere down under
the hood. Got it checked by dealers/mechanics for 3 times. They all say "no
problem." or "normal". I know they must have checked cat convert and vacuum
leak etc. But the hissing has been louder and louder when accelerating even
at very low speed. Another symptom is poor acceleration and a little worse
gas mileage together with the hissing noise coming up a few months ago.

I'm just wondering if this could possibly be due to a leak somewhere in the
emission system other than a plugged catalytic converter? a loose joint in
exhaust? The noise does sound like air/water spraying out from a high
pressure pipe. Would it be wise to get the car back to dealer again, and
just ask for a check on exhaust?

Thanks!